The present invention generally concerns capacitors and more particularly, the organization of internal electrodes within a capacitor having a high voltage breakdown rating.
The present invention relates to the placement of internal electrodes within a multi-layer capacitor made of a dielectric material such as a ceramic dielectric material. Capacitance between spaced-parallel plate regions is a function of their separation. Further, plate density cannot be particularly high in a multi-layer capacitor that relies on only a relatively thin ceramic layer to limit the breakdown voltage. Metal plate regions of alternating polarity are stacked in a parallel relationship and partially overlap each other. The metal plate regions are parallel and overlapping so as to create capacitance along the elementary model of two parallel plate electrodes. The formula for the capacitance of the conventional parallel-plate ceramic capacitor is:
Cap=kA/d
where
Cap is the capacitance in farads,
k is the dielectric constant in farads per meter,
A is the area of electrode overlap in square meters, and
d is the distance of separation between plates in meters.
Although d would desirably be minimized for greatest capacitance, in high voltage capacitors, d cannot be indefinitely small or else the capacitor will be subject to failure from voltage breakdown of the insulating ceramic dielectric. For example, referring to FIG. 4, a known capacitor 10 having a high voltage breakdown rating has a substantially monolithic thee-dimensional body 12 comprised of layers of dielectric material 14. Conductive first electrodes 16 are placed on a first layer of dielectric material 15 and are connected to a conductive first contact 18 on an external portion of the body 12. Conductive second electrodes 20 are also placed between the same layer of dielectric material 15 and are connected to a conductive second contact 22 on another external portion of the body 12. A conductive third electrode 24 is placed on a different, second layer of dielectric material 26. The third electrode 24 is not electrically connected to either of the contacts 18, 22 and overlaps with both the first and second electrodes 16, 20. Referring to FIG. 4B, a first capacitor 28 is formed between the first and the third electrodes 16, 24, and a second capacitor 30 in a series circuit with the first capacitor 28 is formed between the second and the third electrodes 20, 24.
A typical ceramic dielectric will have a voltage rating of 100 volts per mil (0.001 in.) thickness. For example, if the capacitor 10 is designed to have an operating voltage of about 2,000 volts, an axial plate separation, that is, the thickness t of the ceramic layer 15 must be about 10 mils.
Another aspect of high voltage ceramic capacitor design relates to the distance d1 of separation between electrodes 16, 20 of opposite polarity. The plate separation d1 should be 50% greater than the layer thickness and hence the electrode separation t. This is because a voltage breakdown is more likely to occur along the unavoidable imperfections of the seams 32 between the layers 15, 17. Thus, the distance d1 should be about 15 mils, that is, 1.5xc3x9710 mils.
Capacitors so constructed use high voltages, commonly about 750 volts. When the electrodes of the capacitor are subjected to high voltages, for example, on the order of hundreds and, with safety margins, even thousands of volts, the seam 32 is subject to developing voltage breakdown paths between the electrodes 16, 20.
Thus, there is a need for an improved multilayer high voltage ceramic capacitor that has a substantially higher breakdown voltage rating.
The present invention provides a multi-layer capacitor that has a significantly higher voltage breakdown threshold than known capacitors of comparable size. The multi-layer capacitor of the present invention is especially useful in applications where higher voltages may be expected and thus, can be used in a wider range of more rigorous applications than known comparable capacitors. The multi-layer capacitor of the present invention has a construction that substantially strengthens potential voltage breakdown paths within the capacitor and thus, provides capacitors having operating voltages ranging from about 1,000 volts to 10,000 volts and higher.
According to the principles of the present invention and in accordance with one embodiment, the present invention provides a multilayer capacitor having a substantially monolithic body made of layers of dielectric material with first and second external contacts located on the body. A first electrode connected to the first contact is located on a first layer of dielectric material within the body, and a second electrode connected to the second contact is located on a second layer of dielectric material different from the first layer. The first and second electrodes are nonoverlapping with each other. A floating electrode not electrically connected either of the contacts is located on a third layer of dielectric material different from the first and second layers. The floating electrode overlaps the first and second electrodes and forms serially connected capacitors therewith. Locating the electrodes on different layers of dielectric material provides the multilayer capacitor with a higher voltage breakdown threshold than known capacitors of comparable size.
In one aspect of this invention, additional floating electrodes are located on different layers of dielectric material and provide additional serially connected capacitors to increase the voltage breakdown threshold of the multilayer capacitor.
These and other objects and advantages of the present invention will become more readily apparent during the following detailed description taken in conjunction with the drawings herein.